Wafer head template for chemical mechanical polishing and a method for its use

ABSTRACT

The present invention is a planar template for a CMP tool polishing head possessing a means of securing a wafer in CMP polishing where the back surface of the template is held to the polishing head by retaining means and a method for using the planar template.

FIELD OF THE INVENTION

The present invention relates to a wafer template for chemicalmechanical polishing and a method for its use.

BACKGROUND OF THE INVENTION

The present invention is a planar template for a CMP polishing toolpossessing a means of securing a wafer in CMP polishing wherein the backsurface of the said template is held to the polishing head by retainingmeans.

Chemical Mechanical Polishing (CMP) polishing tools, slurry, polishingheads, polishing pads and diamond conditioner disks form the keycomponents of the equipment used to carry out CMP processes in recentyears. These various pieces of equipment have been produced and marketedby several vendors to standards of reliable quality and effectiveness.The function of the slurry is to deliver continuously the mechanicalabrasive particles and chemical components to the surface of the waferand to provide a means of removing reaction products and wafer debrisfrom the polishing surface.

The function of the polishing pad is to abrade the surface of the waferduring polishing in concert with the slurry. The polishing pad istypically made of polyurethane or a substance of similar properties andthe surface is roughened to enhance its effectiveness as a polishingsurface for the wafer. It is positioned face up both to hold the slurryand to facilitate and is rotated from below. The surface of thepolishing pad is usually provided with long grooves that serve both toassist in the dispersion of slurry under the wafer head during polishingand to provide effective removal of spent slurry and wafer detrituswhich are typically swept off of the pad by centripetal force as the padrotates. The pad rotates at a rate of typically about 20 to 100 RPM

The diamond conditioner discs serve the purpose of continual andconsistent roughening of the polishing pad and are suspended from an armor a bridge, platform or similar structure of the polishing tool so thatthey are pressed diamond-bearing face down into the polishing pad.Diamonds on the conditioner disc surface cut and roughen the pad duringCMP operation.

This is necessary because the action of the slurry and the wafer on thepad quickly smooth the pad otherwise greatly diminishing itseffectiveness and the rate of removal of the wafer surface duringpolishing. The diamond conditioner disk sits under a load on thepolishing pad and is both rotated and moved back and for the between thecenter of the pad to ensure an even dispersal of roughening over thepolishing pad.

The polishing head consists an apparatus for holding, supporting androtating the wafer and is hung from a supporting arm, bridge, platformor similar structure of the polishing tool and holds the wafer face downon the face of the polishing pad while applying pressure and rotatingit.

Depending upon the type of wafer to be polished and the specificobjectives of the operator, a variety of loads, respective rotationrates and, in the case of the diamond conditioner disc, motions acrossthe polishing pad surface may be employed. Likewise, the type, quantityand concentration of slurry may be varied to obtain different results.

The polishing head itself is a structure attached to the shaft thatdescending from the apparatus, resting on a bridge or platform typicallypart of the framework of the CMP polishing tool. The polishing head issuspended above the polishing pad except during polishing when it isrotated at between typically 20 and 100 RPM and lowered onto the padunder a load that may be controlled by the operator.

The face of the polishing head is typically a disc or membrane with aflat downward facing surface that holds the wafer by some means. Thesurface may be composed of a number of materials including ceramic orrubber plates or sheets. Where ceramic plates have been used it has beenpossible to equip the plate with a large number of small holes passingthrough it and to install a vacuum device behind the plate. FujikoshiMachinery Corporation has been able to develop this kind of ceramicplate faced vacuum-backed polishing head for the polishing of siliconwafers. However, for the polishing of silicon dioxide and metal layers,this method of polishing has demonstrated problems with the ability tohold the wafer firmly to the wafer head surface.

To overcome this problem, Fujikoshi Machinery Corporation developed atemplate in the shape of a disc made of a single piece of PVC andmachined so that there was a raised ring around the upward facingcircumference of the template of a half centimetre to a centimetre or soin width and a height a half centimetre to a centimetre or so. Thisallowed them to firmly fit the plastic PVC template on the wafer headand it was effectively held with the vacuum delivered through pores inthe ceramic plate. The silicon dioxide or other non-silicon type CMPwafers were affixed to the front of the template using a wet, porouspolymeric backing film with a retaining ring. The raised ring serves thedouble purpose of holding the template in place and preventing slippageand keeping slurry from seeping back behind the template and into thevacuum system. This template with retaining ring design has the furtherquality that is easily removable and allows quick and efficient removalof and attachment of different wafers to the polishing tool resulting ina very considerable time savings and increase in process costeffectiveness.

However, these templates have the problem of being unable to deliveryuniformity of polishing primarily due to the physical properties of thePVC material used. The PVC is machined into the desired shape using alathe which tends to result in slightly thicker center that in turndistorts the wafer surface as it is pressed against the polishing padleading to unacceptable levels of radial non-uniformity in polishing anda significantly flawed final product.

HOW THE PRESENT INVENTION OVERCOMES THE PROBLEMS OF THE PRIOR ART

For CMP polishers using ceramic plates of this type which are designedto hold the wafer in place by vacuum, the present invention avoids theproblems of the prior art by using a template prepared from a stable andinitially very flat material such as polycarbonate sheet which maintainsa very smooth surface and precise thickness even under the conditions ofCMP polishing and maintains the position of the wafer on the lowerdownward facing surface of the template of the present invention bymeans of a commercially available polymeric wafer backing film. Thesurface of the template can be further lapped or polished to achieve aneven greater degree of flatness and it is easily possible to achieve thedesired degree of flatness without machining. Additionally thedimensionally stable material used in the present invention is lesslikely to deform under stress than PVC which leads to even more stablesupport for the wafer on the wafer head. The critical part of the designis that the plate that holds the wafer backing film is made from amaterial that is manufactured with precisely controlled thicknessuniformity. Thickness uniformity of the plate is the most criticalelement for obtaining polish rate uniformity on suitable polishingtools.

The template of the present invention is held in place not only byvacuum which holds it tightly against the said ceramic plate but also bya retaining ring attached to the said template which both serves to keepthe template and by extension the wafer from slipping on the ceramicplate and also prevents slurry from coming behind the template andentering the vacuum holes. The retaining ring can be prepared from thesame material as the template itself and if suitably sealed and attachedto the template with means such as pins and adhesive can as effectivelymaintain the template position and prevent infiltration of the vacuumsystem with excess slurry as the templates of the prior art. Thus thetemplates of the present invention possess all of the advantages of thetemplates of the prior art but provide substantial improvement over theprior art from the standpoint of uniformity of polishing of the wafersurface.

SUMMARY OF THE PRESENT INVENTION

The present invention relates to a planar template for a CMP toolpolishing head possessing a means of securing a wafer in CMP polishingwherein the back surface of the said template is held to the polishinghead by retaining means. The present invention more particularly relatesto such a planar template wherein the size of the template is the samesize or larger than the size of the wafer backing film, the means ofholding the wafer comprises a wafer backing film, the material fromwhich the template is made is polycarbonate sheet, the retaining meansis a vacuum, and a retaining ring is also affixed around the edge of thesaid template on the face toward the polishing head to secure thetemplate in place.

The present invention further relates to a method for CMP polishingusing a planar template for a CMP tool polishing head possessing a meansof securing a wafer in CMP polishing wherein the back surface of thesaid template is held to the polishing head by retaining means. using aplanar template with a means of securing a wafer in CMP polishingwherein the back surface of the said template is held to the polishinghead by retaining means and more particularly to a method for using sucha planar template wherein the size and shape of the template are thesame as the size and shape of the wafer backing film, the means ofsecuring the wafer comprises a wafer backing film, the material fromwhich the template is made is polycarbonate sheet, the retaining meansis vacuum and a retaining ring is affixed around the edge of the saidtemplate on the face toward the polishing head to hold the template inplace.

Finally, the present invention refers to a such a method for using aplanar template wherein the diameter of the wafer is half or less thanthat of the template and two or more wafers may be attached to the sametime to the same template and polished.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view and cross sectional view of the template of thepresent invention.

FIG. 2 is a cross sectional view of the template on the polishing headwith wafer attached.

Unless otherwise indicated, in the Figures like numbers denote the sameelements throughout the Figures

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The template and related methods of use thereof of the present inventionhave been developed in response to the present state of the art, and inparticular, in response to the problems and needs in the art that havenot yet been fully solved by currently available wafer supportingremovable templates for CMP polishing in CMP polishers with vacuumsupplied polishing heads. Thus, it is an overall objective of thepresent invention to provide highly dimensionally stable and flat vacuumsupported CMP polishing head templates for CMP that are easily removableand still effectively prevent slurry from entering the vacuum system andrelated methods that remedy the shortcomings of the prior art.

The purpose of this device and method are to allow more effective CMP ofwafers on CMP polishers equipped with vacuum in the wafer head and inparticularly to substantially improve and render practical in anindustrial or research setting the level of polishing uniformity of thewafer surface possible with the use of such a template.

Furthermore, this improved template may be easily and cost-effectivelyapplied to facilitate and enhance the use of CMP polishing devices andthe polishing heads used in them. These and other features andadvantages of the present invention will become more fully apparent fromthe following description and appended claims, or may be learned by thepractice of the invention as set forth hereinafter.

All dimensions for parts in the present invention follow are based on awafer size of about 200 mm to 300 mm in diameter and may be altered asneeded in proportion to changes in the size of the wafers used. Thespecific dimensions given herein are in no way limiting but are by wayof example to demonstrate an effective embodiment of the invention.

As shown in FIG. 1, the template (8) of the present invention comprisesa hard flat sheet of material (10). The material that may be used is notparticularly limited but it should be hard, dimensionally stable underthe conditions of operation of CMP and resistant to the chemicalcomposition of the slurries used in CMP. Any hard material may be usedas the material for the sheet (10) of the present invention but hardplastic is preferred. Any hard plastic sheet such as acrylic,polyurethane or polycarbonate sheet may be used but polycarbonate sheetis preferred for its availability, cost effectiveness, toughness,flatness and dimensional stability.

The thickness of this material is not particularly limited but it ispreferably between 1/16^(th) inch and ¼ inch and more preferably between0.093 inch and 0.17 inch. The outer shape and dimensions of the sheet(10) are not particularly limited and any sheet dimensions and shapethat do not interfere with the normal operation of the polishing headmay be used but a circular shape is preferred and the diameter thereofshould be sufficiently large to encompass the outer dimensions of theceramic plate at the bottom of the polishing head by at least about ¼inch and more preferably by ½ inch or more. It is preferred that thediameter of the sheet (10) not exceed the diameter of the template bymore than 2 inches.

Around the circumference (12) of the sheet (10) on the upward facingface of the sheet (10) is attached the retaining means. The retainingmeans is not particularly limited and may comprise a series ofarchitectural supports around the circumference of the sheet placed soas to prevent the template from slipping during CMP polishing or acomplete retaining ring with an inner diameter equal to or no more than⅛ inch larger than the diameter of the ceramic plate on the lower faceof the polishing head but a retaining ring (14) is preferred. Theretaining ring (14) may be made of any hard material but hard plastic ispreferred. Any hard plastic sheet such as acrylic, polyurethane orpolycarbonate sheet may be used but polycarbonate sheet is preferred.The sheet (10) and the retaining ring (14) may be of the same ordifferent material and the use of the same material is preferred. Thedegree of flatness of the sheet (10) is not particularly limited.

The dimensions of the retaining ring (14) are not particularly limitedbut it is preferred that the outer edge (16) of the retaining ring beconcentric with the sheet (10) where the sheet (10) is circular in shapeand it is also preferred that the said outer edge (16) be the samediameter as the outer edge (18) of the sheet. The inner edge (20) of theretaining ring (14) should be concentric with the center of the sheet(10) and should be the same diameter as the polishing head or up toabout ⅛ inch in diameter larger and a diameter as close in size aspossible to the size of the plate on the lower face of the polishinghead is preferred. The thickness of the retaining ring of the presentinvention is not particularly limited but must be sufficient to allowthe retaining ring to hold against the sides of the polishing headsecurely and is preferably no thicker than the sheet (10), morepreferably approximately half the thickness of the sheet (10) and isfurther preferably between 0.093 inches and 0.17 inches in thickness.

The means of attaching the retaining ring (14) of the present inventionto the sheet (10) is not particularly limited and any suitable meansincluding bolting, pinning, lamination with adhesive or mechanicalattachment may be used but secure attachment with adhesive and by aseries of pins (34) in recessed openings (36) in both the retaining ring(14) and the sheet (10) supplied for this purpose is preferred,combinations of methods of attachment may be used, and the use ofadhesive between the retaining ring (14) and the sheet (10) in additionto bolting or pinning is more preferred. If pins are used any number ofpins may be used but between 10 and 20 stainless steel, aluminium orbrass pins is preferred.

The wafer is secured to the sheet by means of a backing film (22) with aring (24) for securing the lateral edges of the wafer. Any suitablebacking film commercially available for CMP polishing may be used forthis purpose.

Drawing 2 shows the ceramic plate (26) on the lower face of thepolishing head (28) and the affixation of the template (8) thereto. Thewafer (30) is attached to the backing film (22) by liquid capillaryforces within the ring (24) for securing it on the side opposite theaffixation of the retaining ring and the side of the template possessingthe retaining means is placed on the polishing head (28) so that thesheet (10) face contacts the face of the ceramic plate (26). Vacuum isintroduced through the multiple small openings (not shown) in thesurface of the ceramic plate (26) equipped for that purpose and thetemplate is affixed and ready to commence CMP polishing on the CMPpolishing pad (32). To remove the template, maintenance of the vacuum tothe ceramic plate (26) is discontinued after the completion of polishingoperations and the template (8) with the wafer (30) attached by means ofthe backing film (22) and the ring (24) for securing the wafer isremoved from the CMP polisher.

The CMP polisher of the present invention with a polishing head that isequipped with vacuum to hold a wafer directly or a disc during polishing(not shown) is not particularly limited and CMP polishing tools such asthe APD 500 or APD 800 may be used.

PRACTICE EXAMPLES Example 1

A circular polycarbonate sheet of thickness 0.17 inches was cut from GELexan® to a diameter of 8 and 59/64 inches. To this sheet was affixed aring made of the same material having the same external diameter, aninternal diameter of 8 and 9/64 inches for a width of 25/64 inches and athickness of 3/32^(nd) of an inch. This was affixed to the sheet with 16brass pins ⅛ inch in diameter and 0.2 inches long set in both holesdrilled in the sheet and in the retaining ring at equidistant pointsaround the ring halfway between the inner and outer diameter thereof andset in the holes with Loktite super glue (cyanoacrylate glue). The pinsfully penetrate the ring but do not fully penetrate the sheet. The ringwas further affixed to the sheet by lamination with Loktite super glue(cyanoacrylate glue).

A backing film was attached centered to the side of the template thatdid not possess the retaining rings and a wafer secured therein. Thewafer was then affixed with the retaining ring side facing to theceramic plate head of an Araca APD 500 polisher, the vacuum of thepolishing head was engaged and the wafer and template were secured tothe polishing head.

The unexpected and superior results achieved by means of the presentinvention were a substantial improvement in uniformity of the polishingof the surface of the wafer during the CMP processes using theaforementioned CMP polishing tool.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1

FIG. 1 is a top view and cross sectional view of the template of thepresent invention.

(8) is the entire template

(10) is a sheet

(12) is the circumference of the sheet

(14) is the retaining ring.

(16) is the outer edge of the retaining ring

(18) is the outer edge of the sheet

(20) is the inner edge of the retaining ring

(22) is the backing film

(24) is the ring for securing the wafer

FIG. 2 is a cross sectional view of the template on the polishing headwith wafer attached.

(26) is the ceramic plate on the lower face of the polishing head

(28) is the polishing head

(30) is the wafer

(32) is the CMP polishing pad

(34) are the pins

(36) is the recessed opening for the pins.

EFFECTS OF THE INVENTION

The present invention has the effect of unexpectedly improving theuniformity of the polishing of the wafer surface by providing a muchflatter and more stable surface. Since material with this degree ofplanarity is readily and cheaply available it provides a cheap andeffective way to move various wafers on and off of the CMP polishingtool quickly and efficiently.

1. A planar template for a CMP tool polishing head possessing a means ofsecuring a wafer in CMP polishing wherein the back surface of the saidtemplate is held to the polishing head by retaining means.
 2. A planartemplate according to claim 1 wherein the size and shape of the templateare the same as the size and shape of the wafer backing film.
 3. Aplanar template according to claims 1 wherein the size of the templateis larger than the size of the wafer backing film.
 4. A planar templateaccording to claim 1 wherein the means of holding the wafer to thetemplate comprises a wafer backing film.
 5. A planar template accordingto claims 1, 2, 3, and 4 wherein the material from which the template ismade is hard, flat, smooth plastic sheet.
 6. A planar template accordingto claims 1, 2, 3, and 4 wherein the material from which the template ismade is polycarbonate sheet.
 7. A planar template according to claim 1wherein the retaining means holding the template to the polishing headis vacuum.
 8. A planar template according to claim 1 wherein in additionto the wafer retaining means a retaining ring is affixed around the edgeof the said template on the face toward the polishing head to hold thetemplate in place.
 9. A planar template according to claim 1 wherein thesize and shape of the template are the same as the size and shape of thewafer backing film, the means of securing the wafer to the templatecomprises a wafer backing sheet, the material from which the template ismade is polycarbonate sheet, and the retaining means is vacuum and aretaining ring affixed on the face toward the polishing head to hold thetemplate in place.
 10. A planar template according to claim 1 whereinthe size of the template is larger than the size of the wafer backingfilm, the means of holding the wafer comprises a wafer backing film, thematerial from which the template is made is polycarbonate sheet, theretaining means is a vacuum, and a retaining ring is also affixed aroundthe edge of the said template on the face toward the polishing ring tosecure the template in place.
 11. A method for CMP polishing using aplanar template for a CMP tool polishing head possessing a means ofsecuring a wafer in CMP polishing wherein the back surface of the saidtemplate is held to the polishing head by retaining means using a planartemplate with a means of securing a wafer in CMP polishing wherein theback surface of the said template is held to the polishing head byretaining means.
 12. A method for CMP polishing using a planar templateaccording to claim 11 wherein the size and shape of the template are thesame as the size and shape of the wafer backing film.
 13. A method forCMP polishing using a planar template according to claim 11 wherein thesize of the template is larger than the size of the wafer backing film.14. A method for CMP polishing using a planar template according toclaim 11 wherein the means of holding the wafer comprises a waferbacking film.
 15. A method for CMP polishing using a planar templateaccording to claims 11, 12, 13, and 14 wherein the material from whichthe template is made is a hard, flat, smooth plastic sheet.
 16. A methodfor CMP polishing using a planar template according to claims 11, 12,13, and 14 wherein the material from which the template is made ispolycarbonate sheet.
 17. A method for CMP polishing using a planartemplate according to claim 11 wherein the retaining means is vacuum.18. A method for CMP polishing using a planar template according toclaim 11 wherein in addition to the wafer securing means a retainingring is affixed around the edge of the said template on the face towardthe polishing head to hold the template in place.
 19. A method for usinga planar template according to claim 11 wherein the size and shape ofthe template are the same as the size and shape of the wafer backingfilm, the means of securing the wafer comprises a wafer backing film,the material from which the template is made is polycarbonate sheet, theretaining means is vacuum and a retaining ring is affixed around theedge of the said template on the face toward the polishing head to holdthe template in place.
 20. A method for using a planar templateaccording to claim 11 wherein the size of the template is larger thanthe size of the wafer, the means of securing the wafer comprises a waferbacking film, the material from which the template is made ispolycarbonate sheet, the retaining means is vacuum and a retaining ringis affixed around the edge of the said template on the face toward thepolishing head to hold the template in place.
 21. A method for using aplanar template according to claim 11 wherein the diameter of the waferis half or less than that of the template and two or more wafers may beattached to the same time to the same template and polished.